In the map Archipelago, RAVEN team is spawned on, what we could guess, is a pre-built airstrip. EAGLE team spawns on a combat-ready aircraft carrier (as shown by combat aircraft and helicopters at the ready). The island is/was home to civilians, given the coastal homes on the port.

-RAVEN-
Seemingly, RAVEN has built a FOB, airstrip, and landing zone on the islands. Most likely, the islands were initially in RAVEN control at the conflict's outbreak, and they introduced the landing zone later, alongside a FOB (Likely the FOB was instead upgraded). Raven is already on the island, as opposed to EAGLE who are having to attack.

-EAGLE-
EAGLE obviously is attacking, given a fully manned carrier is off the island's shores, and that they are not in a defensive position. This shows us EAGLE of all teams likely has the most capabilities to go on the offensive, but that is easily disproven, or considered interchangeable between teams.
(More context later)

In the mode, CONQUEST, the player takes command of the EAGLE army to fight RAVEN. What I noticed, and as I'd imagine everyone else who played the mode noticed, is that RAVEN has the most territory compared to EAGLE, with no tiles connecting them yet.


At game start, this (above) is what the map looks like, excluding RAVEN Citadel. In theory, RAVEN is the sovereign owner of these tiles/states given the two forces have not yet met for combat, and RAVEN could not conquer these tiles if they are not in direct combat. It would also make sense that a small nation such as RAVEN would look for expansion/annexation if they had a militant force as strong as what we see here.
At game end, RAVEN will no longer spawn battalions if all previous battalions have been killed, even if they have the full capacity to do so. This can be taken logically, as a misfire in the AI's code, or as I'd like to, as a surrender. No matter how long you wait, RAVEN will not spawn new forces from the Citadel. This could possibly mean the want for a ceasefire, or declaring the Citadel an 'open city', as to deter unnecessary casualties and destruction.

Following the newest update (as of now) with EA21, the SPEC OPS mode has been revamped. As such, we now know that the squad you're commanding, is entirely separate from the other Infantry battalions, and maybe even branch, from the EAGLE armed forces. 'TALON' squadron is a good example as to EAGLE's aggression. TALON has better armor (at least 3 headshots to kill), better equipment (headset radios, night vision systems at the ready, custom kneepads, etc.), and better training given their ability to carry out these missions in different fashion than EAGLE's main infantry. They are designed for warfare. From what we know, RAVEN doesn't have their own spec. ops force, meaning they are largely underequipped for sabotage and assassinations. EAGLE is not.*
(I hadn't played Raven on Spec as of writing this. Raven does have Spec. ops soldiers, though not as equipped as we see with TALON)

Spoilers
Following the events detailed in the HQ's messages, and upon beating RAVEN on COASTLINE, the EAGLE Advisor will fly down in a BLACKHAWK. During the cutscene she gives the player arguably the most powerful weapon in the game: the Railgun. The Railgun is meant to destroy the ever present UFOs during the course of HQ's messages, but it can easily be used as an anti-personel and anti-tank armament. It was made on very short notice, to our knowledge, and boasts the highest reserve ammunition count of any AT designated weapon. Very likely, EAGLE is capable of making much more dangerous weaponry for their troops, and possibly even a WMD (Weapon of Mass Destruction).

A lot of this is hypothetical, or very stretched as to how simple the game is for now, and how little background info we do have. This is all well meant to be fun for the player, and it's as to why the player has so much control, and why they get special weaponry the AI does not.
As a flipside to my initial statement that EAGLE, being 'powerful' as they are might look for expansion, RAVEN may likely be the aggressor given their size compared to EAGLE, thinking they'd be a pushover in war.
On ARCHIPELAGO, the bases could simply be hasty construction. In 1942 and 1943, the Battle of Guadalcanal was fought, in which, Henderson Field was built on very short notice. RAVEN could have either, A: Invaded the islands and taken over these pre-built structures from EAGLE, or B: Built the structures as soon as they landed, such as Henderson Field was to the USMC.
In DUSTBOWL (on Instant Action), the evidently permanent buildings, such as the airbase, are inhabited by EAGLE and not RAVEN. On Conquest, DUSTBOWL isn't even owned by a force yet. Most likely, either force took control of civilian structures for use as a base, or EAGLE is indeed the rightful owner of DUSTBOWL.

Overall it was a thought I had with some evidence to back it up, though a few contradictions. Will we get an answer as to who started the war? Unlikely. Will the war itself end? No.